A vehicle's engine control unit (ECU) is a type of electronic device that controls a series of actuators on an internal combustion engine to ensure the engine runs at its optimal setting. ECUs accomplish this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps (called look-up tables), and adjusting the engine actuators accordingly.
For an engine with fuel injection, the ECU determines the quantity of fuel to inject into the fuel chamber based on a number of parameters. For example, if the throttle position sensor is showing the throttle pedal is pressed further down, a mass flow sensor measures the amount of additional air being drawn into the engine and the ECU will inject a fixed quantity of fuel into the engine. If the engine's coolant temperature sensor is showing the engine has not warmed up yet, more fuel will be injected (causing the engine to run slightly ‘rich’ until the engine warms up). Mixture control on computer controlled carburetors works similarly, but with a mixture control solenoid or stepper motor incorporated in the float bowl of the carburetor.
A special category of ECUs are those which are programmable. These units do not have a fixed behavior and can be reprogrammed, typically by the user. Programmable ECUs are required where significant aftermarket modifications have been made to a vehicle's engine. Examples include adding or changing of a turbocharger, adding or changing of an intercooler, changing of the exhaust system or a conversion to run on alternative fuel. As a consequence of these changes, the old ECU may not provide appropriate control for the new configuration. In these situations, a programmable ECU can be incorporated into the engine's electrical system. These programmable ECUs are programmed/mapped with a laptop or external computer connected using a serial or USB cable, while the engine is running.
The programmable ECU may control the amount of fuel to be injected into each cylinder. This varies depending on the engine's RPM and the position of the accelerator pedal (or the manifold air pressure). The engine tuner can adjust this by bringing up a table on the external computer where each cell represents an intersection between a specific RPM value and an accelerator pedal position. Each cell has a value corresponding to the amount of fuel to be injected. This table is often referred to as a fuel table or fuel map.
By modifying these table values while monitoring the exhausts using a wide band lambda probe to see if the engine runs rich or lean, the tuner can find the optimal amount of fuel to inject to the engine at every different combination of RPM and throttle position. Other parameters that are often mappable are:
Ignition Timing: Defines at what point in the engine cycle the spark plug should fire for each cylinder.
Rev. limit: Defines the maximum RPM that the engine is allowed to reach.
Water temperature correction: Allows for additional fuel to be added when the engine is cold, such as in a winter cold-start scenario or when the engine is dangerously hot, to allow for additional cylinder cooling.
Transient fueling: Instructs the ECU to add a specific amount of fuel when throttle is applied (“acceleration enrichment”).
Low fuel pressure modifier: Instructs the ECU to increase the injector fire time to compensate for an increase or loss of fuel pressure.
Closed loop lambda: Allows the ECU to monitor a permanently installed lambda probe and modify the fueling to achieve the targeted air/fuel ratio desired. This is often the stoichiometric (ideal) air fuel ratio, which on traditional petrol (gasoline) powered vehicles this air:fuel ratio is 14.7:1. This can also be a much richer ratio for when the engine is under high load, or possibly a leaner ratio for when the engine is operating under low load cruise conditions for maximum fuel efficiency.
Some of the more advanced standalone/race ECUs include functionality such as launch control, operating as a rev limiter while the car is at the starting line to keep the engine revs in a ‘sweet spot’, waiting for the clutch to be released to launch the car as quickly and efficiently as possible. Other examples of advanced functions are:
Wastegate control: Controls the behavior of a turbocharger's wastegate, controlling boost. This can be mapped to command a specific duty cycle on the valve, or can use a PID based closed-loop control algorithm.
Staged injection: Allows for an additional injector per cylinder, used to get a finer fuel injection control and atomization over a wide RPM range. An example being the use of small injectors for smooth idle and low load conditions, and a second, larger set of injectors that are ‘staged in’ at higher loads, such as when the turbo boost climbs above a set point.
Variable cam timing: Allows for control variable intake and exhaust cams (VVT), mapping the exact advance/retard curve positioning the camshafts for maximum benefit at all load/rpm positions in the map. This functionality is often used to optimize power output at high load/rpms, and to maximize fuel efficiency and emissions as lower loads/rpms.
Gear control: Tells the ECU to cut ignition during (sequential gearbox) upshifts or blip the throttle during downshifts.
A racing vehicle's ECU is often equipped with a data logger that records all sensors for later analysis using special software in a PC. This can be useful to track down engine stalls, misfires or other undesired behaviors during a race by downloading the log data and looking for anomalies after the event. In order to communicate with the driver, a race ECU can often be connected to a “data stack”, which is a simple dash board presenting the driver with the current RPM, speed and other basic engine data. These race stacks, which are almost always digital, talk to the ECU using one of several proprietary protocols running over RS232 or CANbus, connecting to the DLC connector (Data Link Connector) usually located on the underside of the dash, inline with the steering wheel.
Presently, active tuning requires external computing power such as a laptop to reset the engine's values. Obviously, this process is inefficient and cannot meet the demands of high performance vehicles such as racing vehicles and performance sports cars. There are various companies in the market that cater to active tuning, including PCLink (www.linkecu.com) & EFILive (www.efilive.com), both of which require PCs or laptops that are separate from the vehicle to perform auto tuning for fuel tables. Other companies, such as Motec, use internal active tuning, but the market lacks a system and methodology for using internal (i.e., ECU) based active tuning on an OEM engine control unit.